This invention relates generally to amplifiers used to measure low level signals and, more particularly, to low-noise amplifiers used to measure ultra low-frequency signals where direct current (dc) blocking between the signal source and input to the low-noise amplifier is required. Typically, such requirement arises when large dc voltages are present at the signal source.
Low-noise amplifiers, which are also referred to as “LNAs”, are a common and useful building block in electronic circuits where one of the design goals is the selective amplification of very small analog signals that are present within a noisy common mode signal typically created by power line (60 Hz noise), and where both the signal and common mode noise occupy the same frequency band of operation.
Design specifications that require the detection of very low-frequency signals with large DC offsets are typically satisfied by using very large valued capacitors and resistors between the signal source and the LNA to achieve the desired circuit response characteristics. Large valued capacitors and resistors can severely hamper the integrated circuit implementation of a LNA due to the large integrated circuit (IC) micro chip or die area and geometry dedicated to creating such capacitors and resistors.
As is understood, bio-potential signals such as electromyograms, electroneurograms and low-field potentials from cortical activity have an extremely low-frequency content in their signals. Typical frequency ranges are 0.5 Hz to 1 kHz. The requirement for dc-blocking of leakage currents to protect tissue or to block high-voltage (12 to 14V) stimulation voltages that may reside on the same sensing terminals, requires that the sensing amplifier be ac-coupled to the sensing terminals. When ac-coupling is required in an integrated circuit implementation, typical implementations result in the waste of large die area due to the requirement of extremely large capacitors and resistors that form a high-pass filter function. Bio-signal levels of under 10 uVrms discourage the use of active filter implementation, as the noise contributions would be too large. An embodiment of the present invention discloses a novel and simple method to achieve dc-blocked, ultra low-frequency (ULF) low-noise amplifier (LNA) structure. The implementation achieves high-pass filter corners of under 20 mHz (millli-Hertz) with die areas that are 50 times smaller than conventional approaches. The approach disclosed uses passive capacitive components that further ensure that the low-noise characteristics are maintained or even improved over state-of-the-art approaches. Each of the signals identified possesses quite different amplitude and frequency characteristics as shown in Table 1.
TABLE 1Biosignal Types And Corresponding Electrical TraitsSignal TypeAmplitude RangeFrequency RangeElectromyogram (EMG) 10 μV-10 mV 10 Hz-3 kHzNeural potentials 10 μV-500 μV100 Hz-5 kHzLow field potentials (LFPs)100 μV-1 mV 0.5 Hz-50 Hz
Furthermore, the presence of low-frequency signals typically necessitates the use of input devices that consume large IC die areas in order to lower the “1/f” noise of the LNA. Several “1/f” noise reduction techniques are known in the art. While these techniques are beneficial in reducing low-frequency noise and circuit IC die area, they are less practical when the size and power constraints are especially stringent and when a dc-blocking capacitor is required in the circuit design.